Forever summer: D.C. could near 90 Thursday and 80s could extend through next week

In an ordinary beginning to October around Washington, high temperatures are falling back toward 70 degrees while the nights cool through the 50s. The air is crisp, we’re wearing fleece and sipping pumpkin spice lattes and our air-conditioners are shut down until late spring.

But not this October, in which summer — which refused to go away in September — obstinately persists. Don’t stow away the shorts and sandals just yet.

Temperatures are strangely trending up during the month’s opening stanza. Over the next 10 days, they’ll frequently surpass 80 degrees and could even flirt with 90 on one or two days.

Both the National Weather Service’s 6-to-10- and 8-to-14-day outlooks are calling for above-normal temperatures with a high degree of confidence.

So exactly how warm are we talking?

On Thursday, high temperatures are forecast to reach the mid- to upper 80s. Even 90 isn’t entirely out of the question. Even so, the high temperature probably won’t come close to the record, which is 94 degrees set in 1954. Factoring in the humidity, though, it might feel that hot.

Heat index forecast at 4 p.m. Thursday from HRRR model.

The warm weather, projected to extend through October’s second week, is likely to end up being most remarkable for its duration.

After a brief cool-down with highs in the 70s Friday and Saturday, temperatures are forecast to catapult back above 80 Sunday and remain 80 or higher for much of next week.

Models vary on the intensity of the heat.

The American model, which tends to have a warm bias, predicts highs in the mid- to upper 80s Sunday through Friday (next week). The European model, which sometimes runs a few degrees too cool, favors low 80s.

Usually, a blend of the two models offers a fairly reliable forecast — so low to mid-80s seems like a reasonable prediction during that Sunday-Friday span. Such temperatures would be 10 to 15 degrees above normal.

There’s a small chance that temperatures flirt with 90 degrees on one or two days next week (if the American model proves more correct). The latest it has ever hit 90 in Washington is Oct. 11, which is next Thursday. There’s an outside chance we could challenge that record.

The fact that September was so swampy reduces our odds of matching or surpassing historical high temperature records. The high amount of soil moisture from almost 10 inches of rain increases the humidity, which makes it harder for the air to heat up. The muggy air will, however, make it harder for temperatures to cool at night — so some record warm nights are possible.

Dew points, an indicator of humidity, are forecast to hover in the mid- to upper 60s for much of next week, which is typical for July — not early October.

The cause of this extended period of anomalous warmth is a strong, persistent area of high pressure at high altitudes. We often refer to such high-pressure cells as heat domes in the summer.

European model forecast of high-altitude weather pattern over the next 10 days. It shows a large high-pressure cell parked over the eastern United States, which will lead to abnormally warm temperatures.

Model forecasts show this high-pressure zone remaining stuck over the region for the next 10 days, after which it should slowly weaken and retreat.

But at its peak intensity early next week, this high-pressure zone is predicted to rank among the strongest on record for this time of year in the Mid-Atlantic.

Statistical analysis of rarity of high-pressure cell at high altitudes (500 millibars, or around 18,000 feet) projected by American GFS model next Monday. (Tomer Burg)

Another historically intense high-pressure zone, parked over Alaska in recent weeks, has resulted in record warmth in that region. Available data show these high-pressure ridges have trended more intense in recent decades, probably because of climate change.

After this weather pattern finally relents, maybe by October’s third week, it really should start to feel more like fall.

Capital Weather Gang reader Josh Lorenzo recently wrote a humorous piece in which September interviews to become the fourth month of summer. Maybe the first half of October should send in its résumé to join the season, as well?

Jason SamenowJason Samenow is The Washington Post’s weather editor and Capital Weather Gang's chief meteorologist. He earned a master's degree in atmospheric science and spent 10 years as a climate change science analyst for the U.S. government. He holds the Digital Seal of Approval from the National Weather Association. Follow